There are many circumstances in the oil industry where it is desirable to cut into or through a downhole object within a well. A typical application is in the cutting of downhole tubular goods within a well. For example, in the course of drilling a well, the drill pipe may become stuck at a downhole location. This may result from "keyseating" or as a result of cuttings which settle within the well around the lower portion of the drill string. In order to remove the drill string from the well, it may be necessary to sever the drill pipe at a location above the stuck point. Similarly, it is often necessary to carry out downhole cutting operations during the completion or operation of oil or gas wells. For example, it is sometimes desirable to sever casing or tubing at a downhole location in order to make repairs or to withdraw the tubular goods from a well which is being abandoned. In other circumstances it is desirable to cut slots, grooves or perforations in downhole tubular goods. Thus, it is a common expedient to perforate the casing and surrounding cement sheath of a well in order to provide fluid access to a hydrocarbon bearing formation. Similarly, it is sometimes desirable to perforate tubing in the completion or recompletion of a well.
While mechanical means may be employed to cut downhole objects, this is often accomplished through chemical cutting techniques. Shaped charges may be employed to perforate or sever tubular goods within the well. Another technique involves the application of a chemical cutting agent which cuts through objects in the well by direct chemical reaction. For example U.S. Pat. No. 2,918,125 to Sweetman discloses a downhole chemical cutter which employs cutting fluids that react violently with the object to be cut with the generation of extremely high temperatures sufficient to melt, cut, or burn the object. Examples of cutting agents disclosed in Sweetman are fluorine and the halogen fluorides including such compounds as chlorine trifluoride, chlorine monofluoride, bromine trifluoride, bromine pentafluoride, iodine pentafluoride and iodine heptafluoride. The cutting fluid is expelled from the tool through radial ports in jet cutting streams. The attendant reaction is highly exothermic and the tubing, drill pipe, etc. is rapidly penetrated. In Sweetman, the cutting ports extend radially from a central bore within the discharge head of the cutting tool which terminates in a reduced diameter bore which is open to the lower or front end of the cutting tool. The reduced diameter bore is internally threaded to receive a threaded plug which closes the lower end of the bore.
Various means may be employed to anchor the cutting tool at the desired location within the well. This is particularly important when, as in the case of the Sweetman apparatus, the cutting tool is run into the well on a wire line. Thus, in Sweetman, the cutting tool may be anchored by means of bow-spring mounted slips which, upon relative movement between the slip cage and a frusto-conical mandrel, are displaced outwardly into gripping engagement with the surrounding wall structure. Another technique disclosed in the Sweetman patent and also in U.S. Pat. No. 4,180,131 to Chammas employs fluid pressure from a suitable source to both activate the anchoring means and to dispel the chemical cutting fluid from the tool against the surface to be severed or otherwise cut. For example, in the Chammas patent, a cutting tool is disclosed in which gas from a propellant charge displaces a piston against the action of a compression spring to cam one or more wedges outwardly against the tubing string to be cut. The gas from the propellant charge is also employed to force the chemical cutting fluid, preferably bromine trifluoride, into contact with a preignitor e.g. steel wool, and thence downwardly through the bore of a severing head. The severing head bore is open to the front end of the severing head and contains a pressure transmitting fluid which is retained in place by a diaphragm that is ruptured when the tool is fired. A plurality of radial discharge ports extend from the interior bore of the severing head to the exterior of the tool. A piston is disposed in the bore adjacent these cutting ports. When the tool is fired, the chemical agent under pressure forces the piston downwardly into a reduced section of the cutting head bore where it is held in place. In this position, the piston is below the cutting ports and the chemical cutting fluid is forced outwardly through the ports and against the tubular goods to be cut.
Particularly suitable chemical cutting tools are disclosed in U.S. Pat. Nos. 4,345,646 to Terrell and 4,415,029 to Pratt and Terrell, the inventors herein. In these tools, which employ a halogen fluoride such as bromine trifluoride as the chemical cutting agent, a chemical module assembly is located intermediate a propellant and slip assembly and a discharge head assembly. Gas pressure generated by the ignition of a propellant charge is employed to rapidly move a slip array against a conical mandrel to expand the slip array. During this time the cutting action takes place. The slip array is then rapidly retracted by means of a biasing mechanism such as a compression spring. The gas pressure also forces the chemical cutting agent from the chemical module assembly into the discharge head and passed an ignitor such as steel wool. The discharge head is provided with radial discharge ports which are normally closed by a piston. The chemical cutting agent under the applied gas pressure forces the piston downward, thus opening the ports to the cutting agent. The slip array in the Terrell and Pratt et al patents comprises a plurality of slips which are biased inwardly and which comprise several sets of gripping teeth to accommodate use of the tool in tubular goods of different diameters.
Yet another downhole cutting tool which employs gas-forming and non-gas forming pyrotechnic compositions is disclosed in U.S. Pat. No. 4,352,397 to Christopher. In this tool, an ignitor section is provided near the top of the tool followed by a column of non-gas-forming pyrotechnic fuel composition which extends into a first set of solid annular pellets formed of a gas-forming pyrotechnic fuel composition. At the lower end of the tool is a fuel chamber filled with a plurality of stacked annular pellets formed of a gas-forming pyrotechnic fuel composition. A powdered non-gas forming pyrotechnic fuel composition is disposed within the central passage formed by the stacked pellets. Intermediate the fuel chamber and the ignition assembly is a central portion having a plurality of radially extending cutting ports. The non-gas-forming pyrotechnic fuel composition is a mixture of a metal-metal oxide mixture such as "thermite" and the gas-forming fuel composition is a similar metal-metal oxide mixture which also includes polytetralfluoroethylene (teflon) in an amount within the range of about 1 to 60% by weight. Upon ignition of the pyrotechnic fuel formulation within the fuel chamber, the resulting pressure forces the hot reaction products upwardly through the tool into the discharge ports.